Dual-heterogeneity modulates phase transformation and strain partitioning for enhanced hydrogen embrittlement resistance in a metastable ferrous medium entropy alloy

A dual-heterogeneous (DH) microstructure with heterogeneous grain size and dislocation density was fabricated in a Fe 50 Mn 30 Co 10 Cr 10 metastable ferrous medium-entropy alloy (MEA) to address the dilemmas of low yield strength (YS), severe hydrogen embrittlement (HE) and strength-ductility trade-off in conventional MEAs. Compared with the single-heterogeneous (SH) MEA of bimodal recrystallized grains, the DH-MEA achieved a remarkably higher yield strength of ∼755 MPa (vs. ∼480 MPa) with a moderately reduced tensile elongation (TEL), and more importantly, its HE susceptibility was drastically reduced with only ∼18% TEL loss (vs. 48% for SH-MEA). The DH microstructure endows the alloy with enhanced reversible hydrogen trapping to disperse hydrogen atoms, suppresses detrimental martensitic transformation in coarse recovered grains via low hydrogen-trapping capacity. Meanwhile, the DH-induced heterogeneous strain partitioning alleviates interfacial stress concentration and inhibits hydrogen-induced cracking by promoting dislocation accumulation and mechanical twinning. This work provides a novel microstructural design strategy for developing high-performance MEAs with combined high strength and excellent HE resistance.